a) Field of the Invention
This invention relates to potentially hydrophilic resins, compositions comprising the same, and methods of their use.
b) Description of the Related Art
In recent years, plastics are used as products such as containers and wrapping bags in a variety of fields. Many of these products are printed on their surfaces or are provided with sheet-shaped prints adhered on their surfaces. From the viewpoint of low degradability of plastic products in the natural world, resource saving, economy and the like, some of plastic products are recovered by segregation and separate collection and are recycled. However, inclusion of plastic products, which are applied with prints or the like, upon regeneration results in entire or localized coloring of recycled products. Accordingly, these recycled products are substantially reduced in commercial value or may not be reused in many instances. In some instances, detrimental drawbacks may arise in connection with physical properties of recycled products. These plastic products with prints or the like applied thereon as described above are, therefore, simply disposed as are or subjected to thermal or incineration disposal. Environmental contamination by dioxin and the like, which occur upon incineration, has however become a social problem, leading to a strong demand for an advantageous method which permits recovery and reuse of plastic products having prints or the like applied thereon.
With a view to meeting the above-mentioned demand, some methods have been proposed to decolor products, which are colored by prints or the like, upon recovering and reusing plastic products having prints or the like applied thereon. For example, JP 52-26549 B discloses a method, according to which a printed product is decolored by treating it with an organic solvent capable of causing dissolution or swelling of a printed coating applied on the product and separating (peeling) the printed coating from the product. In view of the need for separation and recovery of the organic solvent in liquid waste employed in the decoloration treatment, the danger of flammability of the organic solvent, and the like, the solvent-relying method involves many problems such as the need for the installation of an explosion-proof facility and a safety management. On the other hand, a method that involves treatment of a printed product with alkaline water is simple and economical, but the treatment with alkaline water cannot easily separate printed coatings because printed coatings are often formed with hydrophobic film-forming components such as urethane resins, acrylic resins, cellulose-base resins, rosin or ethylene-vinyl acetate copolymer.
A further method is discussed in JP 58-152913 B. According to this method, an alkaline-condition-soluble resin or a hydrophilic resin is added to a printing ink to be used for printing an article, and the article printed with the printing ink is treated with alkaline water to remove a printed coating from a surface of the article. This method is an excellent method making combined use of physical properties of the printing ink and its separability under alkaline conditions. Nonetheless, this method involves a problem in that the printing ink employed in it is applicable only in a narrow range. For example, the printing ink may undergo flocculation or thickening or may lead to prints reduced in luster and transparency and may hence lack properties appropriate as a printing ink, unless the alkali-condition-soluble resin is added by taking into consideration its compatibility with resin(s) in the printing ink.
Further, when the resin in a printing ink does not have elastic property, prints cannot be readily removed even if an attempt is made to separate them by adding an alkali-condition-soluble resin or a hydrophilic resin. Moreover, fragments so separated turn to fine fragments dispersed in the treatment solution, and for the recovery of such fine fragments so dispersed, use of a costly filter or the like is needed.
To solve the above-described problems of the conventional art, the present invention therefore has as objects thereof the provision of a potentially hydrophilic resin, which makes it possible to remove a coating printed on a plastic product or the like or a sheet-shaped print adhered on the product by treatment with alkaline water, to recover and regenerate the plastic product in a colorless form and further to readily separate the removed printed coating or sheet-shaped print from a treatment solution, a composition comprising the resin, and a method of use of the composition.
The above-described objects can be achieved by the present invention to be described hereinafter. Described specifically, the present invention provides a potentially hydrophilic resin (hereinafter simply called xe2x80x9cthe resin of the present inventionxe2x80x9d) which comprises at least monomer units readily saponifiable under alkaline conditions to form carboxyl groups, carboxyl-containing monomer units, and hydroxyl-containing monomer units, a potentially hydrophilic resin composition (hereinafter simply called xe2x80x9cthe resin composition of the present inventionxe2x80x9d) comprising the resin (specifically, a printing ink, an adhesive, a primer, or the like), and a method of use of the resin composition.
A printing ink comprising the resin of the present invention has various good properties as a printing ink, because it does not contain any water-soluble resin by itself. For the removal of a printed coating applied beforehand to an article with the printing ink, it is only necessary to simply treat the article with alkaline water. As a result, methyl acrylate units in the printed coating are readily saponified with the alkaline water so that the hydrophilicity of the resin making up the printed coating is increased. The printed coating is caused to swell and is readily allowed to separate from the surface of the article. Accordingly, the printed coating is neither dissolved in the alkaline water nor formed into fine fragments but is separated as a film. The printed coating so separated can be separated with ease from the alkaline water employed for its removal.
An adhesive comprising the resin of the present invention does not contain any water-soluble resin by itself so that, when a sheet-shaped print is adhered on an article with the adhesive, the sheet-shaped print has good durability against wetness, dew or the like. For the subsequent removal of the sheet-shaped print, it is only necessary to simply treat the article with alkaline water. As a result, methyl acrylate units in the adhesive coating are readily saponified with the alkaline water so that the hydrophilicity of the adhesive coating is increased. The adhesive coating is caused to swell, and the sheet-shaped print is allowed to readily separate from the surface of the article. Accordingly, the adhesive coating is neither dissolved in the alkaline water nor formed into fine fragments but separates together with the sheet-shaped print. The separated adhesive coating and sheet-shaped print can be separated with ease from the separating treatment solution.
When an article is provided on a surface thereof with a prime coat by using a primer, which comprises the resin of the present invention, and the prime coat is printed on a surface thereof or a sheet-shaped print is adhered on the surface of the prime coat, the print applied or adhered on the prime coat has good durability against wetness, dew or the like because the primer itself does not contain any water-soluble resin. For the subsequent removal of the printed coating or the sheet-shaped print, it is only necessary to simply treat the article with alkaline water. As a result, methyl acrylate units in the prime coat are readily saponified with the alkaline water so that the hydrophilicity of the resin making up the prime coat is increased. The prime coat is caused to swell, and the printed coating or the sheet-shaped print is allowed to readily separate together with the prime coat. Accordingly, the prime coat is neither dissolved in the alkaline water nor formed into fine fragments but separates together with the printed coating or the sheet-shaped print. The separated prime coat and printed coating or sheet-shaped print can be separated with ease from the separating treatment solution.
In the above-described three kinds of methods of use, employment of a resin physically or chemically crosslinked by the use of (a) a macromonomer or (b) a crosslinking agent upon production of the resin of the present invention or (c) formation of a coating such as a printed coating by combined use of a crosslinking agent upon formation of the coating or combined use of the above techniques (a) to (c) can promote the penetrability of alkaline water into a coating formed from the composition when the coating is brought into contact with the alkaline water. Therefore, separation of the above-described various coatings can be promoted further.
The potentially hydrophilic resin, composition and method of use according to the present invention, therefore, makes it possible to remove a coating printed on a plastic product or the like or a sheet-shaped print adhered on the product by treatment with alkaline water, to recover and regenerate the plastic product in a colorless form and further to readily separate the removed printed coating or sheet-shaped print from a treatment solution.
The present invention will next be described in further detail by preferred embodiments.
The resin of the present invention is characterized in that it comprises at least monomer units readily saponifiable under alkaline conditions to form carboxyl groups, carboxyl-containing monomer units, and hydroxyl-containing monomer units.
As a monomer readily saponifiable under alkaline conditions to form a carboxyl group, conventionally-known lower alkyl esters of polymerizable unsaturated carboxyl compounds are all usable although methyl acrylate is preferred for its excellent saponification rate under alkaline conditions. A description will hereinafter be made taking methyl acrylate as a representative example of the monomer readily saponifiable under alkaline conditions to form a carboxyl group.
As a carboxyl-containing monomer, conventionally-known polymerizable unsaturated carboxylic compounds are all usable although acrylic acid or methacrylic acid is preferred for its excellent copolymerizability with methyl acrylate. The description will hereinafter be made taking (meth)acrylic acid as a representative example of the carboxyl-containing monomer.
As a hydroxyl-containing monomer, conventionally-known hydroxyl-containing monomers are all usable. However, hydroxylalkyl (meth)acrylates excellent in the copolymerizability with methyl acrylate, for example, hydroxyalkyl (meth)acrylates which contain ethylene, propylene, butylene and the like as alkylene groups are preferred. The description will hereinafter be made taking hydroxylethyl (meth)acrylate as a representative example of the hydroxyl-containing monomer.
In preferred embodiments of the present invention, macromonomers are also used in combination with the above-described monomers. Conventionally-known macromonomers are all usable as macromonomers insofar as they contain polymerizable double bonds at terminals thereof. Preferred macromonomers can include, for example, macromonomers containing polymerizable groups at terminals of hydrophobic resins and having molecular weights of from 4,000 to 30,000 or so. Illustrative are polystyrene resins and polymethacrylate ester resins (the carbon numbers of ester residual groups: 1 to 17), which have polymerizable groups such a methacryloyl groups at terminals.
In the above description, the respective monomer units may preferably be in such weight proportions as described below when whole monomer units of the resin of the present invention are assumed to be 100 wt. %. Namely, methyl methacrylate may account for 20 to 90 wt. %, more preferably for 30 to 80 wt. %. (Meth)acrylic acid may account for 0.5 to 35 wt. %, more preferably for 1.0 to 20 wt. %. Hydroxyethyl (meth)acrylate may account for 0.5 to 35 wt. %, more preferably for 1.0 to 20 wt. %. Macromonomer units may account for 0 to 30 wt. %, more preferably for 1 to 20 wt. %. Other monomer component or components may account for 5 to 75 wt. %, more preferably for 5 to 50 wt. %.
Described generally, a content of methyl acrylate lower than 20 wt. % may result in a resin a coating of which, even when subjected to saponification treatment, cannot be rendered sufficiently hydrophilic so that its separability from an article is insufficient. A content of methyl acrylate higher than 90 wt. %, on the other hand, may lead to a resin a coating of which, after saponification, is provided with excessively high water-solubility so that penetration of alkaline water into the coating becomes slow and removal of the coating takes time. Contents of methyl acrylate outside the above range are therefore not preferred.
In general, a content of (meth)acrylic acid lower than 0.5 wt. % may result in a resin a coating of which is provided with sufficient hydrophilicity, so that substantial time is needed for the saponification treatment of methyl acrylate and separability of the coating is insufficient. A content of (meth)acrylic acid higher than 35 wt. %, on the other hand, may lead to a coating which is provided with excessively high water-solubility so that problems arise in various basic physical properties of the coating such as waterproofness.
Generally, a content of hydroxyethyl (meth)acrylate lower than 0.5 wt. % may result in a resin, a coating of which takes substantial time for the saponification treatment of methyl acrylate in the coating and has insufficient separability from an article. A content of hydroxyethyl (meth)acrylate higher than 35 wt. % may lead to unstable production of a resin and therefore, may not be practical. Upon production of the resin of the present invention in an aqueous system, hydroxyethyl (meth)acrylate acts as an emulsifier during polymerization, thereby making it possible to obtain a stable ultra-fine dispersion of the resin. When the resin of the present invention is produced in a non-aqueous system, the hydroxyethyl (meth)acrylate units serve as reaction sites for a crosslinking agent upon subsequently crosslinking a coating of the resin with the crosslinking agent.
When a macromonomer is used in combination with the above-described monomer components in the present invention, it may be used in a proportion of from 0.1 to 35 wt. %. A content of the macromonomer higher than 30 wt. % may result in a coating hydrophobicity of which is so high that penetration of water into the coating becomes slow, substantial time is needed for the saponification of methyl acrylate, and significant time is also needed for the removal of the coating. Contents of the macromonomer outside the above range are therefore not preferred. Further, upon production of the resin of the present invention in a non-aqueous system, the macromonomer contributes to the stabilization of a polymerization system, thereby making it possible to obtain the target resin in the form of a high concentration solution.
In the present invention, the total weight of methyl acrylate and (meth)acrylic acid in the resin of the present invention is usually important. When the proportions of the respective monomer units are assumed to fall within the above-described ranges, respectively, and the resin of the present invention is assumed to be 100 wt. %, this total weight may usually range from 25 to 95 wt. %, preferably from 30 to 80 wt. %. A total weight lower than 25 wt. % may result in a resin, which has insufficient hydrophilicity as a whole and provides a coating the separability of which is insufficient even after saponification treatment. A total weight higher than the above range, on the other hand, may lead to a resin, which has excessively high hydrophilicity as a whole. A coating of the resin is conversely inferior in removal rate, and moreover, the coating so removed turns to small fragments to lose its integrity as a removed coating or is dissolved, thereby making it difficult to separate the removed coating from the separating treatment solution.
In the resin of the present invention having the above-described monomer composition, one or more monomers other than those described above may be further copolymerized to control the performance of the resulting resin of the present invention. Examples of such monomers can include lower alkyl esters of (meth)acrylic acid other than methyl acrylate, such as methyl methacrylate, ethyl methacrylate, n-propyl (meth)acrylate, isobutyl (meth)acrylate and tert-butyl (meth)acrylate; dialkyl (C1-C5) maleates and monoalkyl (C1-C5) maleates; (meth)acryl-amides such as N,Nxe2x80x2-dimethyl-(meth)acrylamide and N,Nxe2x80x2-diethyl(meth)acrylamide; mono(meth)acrylates of polyethylene glycols having molecular weights of from 200 to 600; styrenes such as styrene and xcex1-methylstyrene; and vinyl acetate and vinyl chloride. They can be used either singly or in combination and, when the sum of whole monomer units is assumed to be 100 wt. %, can be used in a proportion of from 0 to 50 wt. %, preferably from 10 to 35 wt. % based on the whole monomer units.
In the present invention, it is also possible to further copolymerize a polyfunctional vinyl monomer such that at least a part of the resin of the present invention is crosslinked to adjust the crosslink density of the resulting resin. The polyfunctional vinyl monomer may be added such that the content of polyfunctional vinyl monomer units accounts for 0 to 5 wt. %, preferably 0.1 to 3.5 wt. % of the whole resin. Use of such a polyfunctional vinyl monomer makes it possible to provide the resulting resin of the present invention with an increased crosslinking degree, thereby bringing about a further advantageous effect in addition to an advantageous effect available from the above-described macromonomer.
Illustrative of the polyfunctional vinyl monomer are divinylbenzene, ethylene glycol di(meth)acrylate, polyethylene glycol di(meth)acrylate, propylene glycol di(meth)acrylate, polypropylene glycol di(meth)acrylate, and tri- or tetra(meth)acrylates of polyhydric alcohols. Divinylbenzene is particularly preferred.
The resin of the present invention, which has the monomer composition described above, can be obtained by a conventionally-known polymerization process, for example, by copolymerizing the monomers by emulsion polymerization, soap-free polymerization, dispersion polymerization or the like. The resin of the present invention available as described above may preferably have a weight average molecular weight of from about 10,000 to 100,000, an acid value of from 8 to 250, and a hydroxyl value of from 4 to 140.
The resin composition of the present invention comprises the resin of the present invention as described above, an adhesive component or film-forming component, and a liquid medium in which these resin and component are dissolved or dispersed. Modes of its use can include, but are not limited to, adhesives, printing inks, primers and the like can be mentioned. In general, the resin composition of this invention may have solid contents of from 5 to 75 wt. % when the whole amount of the composition is assumed to be 100 wt. %. When the whole solids are assumed to be 100 wt. %, the potentially hydrophilic resin may account for 10 to 90 wt. %, preferably 20 to 80 wt. %, the adhesive component or film-forming component may amount for 1 to 85 wt. %, preferably 2 to 75 wt. %, and other component(s) may amount for 0.5 to 20 wt. %, preferably 1 to 15 wt. %.
Examples of the adhesive component or film-forming component can include protein, casein, starch and natural rubber; polyisoprene rubber, chloroprene rubber, butyl rubber, styrene-butadiene copolymer rubber, acrylonitrilebutadiene copolymer rubber, acrylic copolymers, vinyl acetate copolymers, vinyl chloride copolymers, ethylenevinyl acetate copolymers, polyurethanes, polyamides, poyesters, polyethylene and polypropylene and chlorinated products thereof, polyisobutyrene, polyvinyl butyral resin, silicone resins, coumarone resins, ketone resins, chlorinated rubber, and cyclized rubber.
Illustrative of the liquid medium, which is adapted to dissolve or disperse the resin of the present invention and the adhesive or film-forming component, are water, methanol, ethanol, isopropyl alcohol, methyl ethyl ketone, methyl isobutyl ketone, toluene, xylene, cyclohexanone, an mixtures thereof.
The above-described resin composition of the present invention can be formulated into printing inks by adding coloring materials similar to those employed in conventional printing inks. Further, addition of adhesive components can formulate it into adhesives. To use it as a primer, it is only necessary to use a suitable film-forming component as a primer component. In addition, plasticizers, such as dicyclohexyl phthalate, dioctyl phthalate, dibutyl phthalate and dibutyl adipate, and other additives can also be added.
A recycling method of the present invention for an article comprises printing or coating the article on a surface thereof with the resin composition of the present invention containing a coloring material (printing ink) and drying the resin composition, and subsequent to use of the article, treating the article, which carries the printed coating formed thereon, with alkaline water to saponify methyl acrylate in the printed coating such that the coating is provided with increased hydrophilicity and is caused to swell, and then removing the coating so swollen from the article.
Another recycling method of the present invention for an article comprises coating the article on a surface thereof with the resin composition of the present invention and drying the resin composition to form a coating, printing desired characters or pattern on a surface of the coating with a desired ink or adhering a sheet-shaped print to the surface of the coating, and subsequent to use of the article, treating the article with alkaline water to saponify methyl acrylate in the coating such that the coating is provided with increased hydrophilicity and is caused to swell, and then removing, from the article, the coating so swollen and the printed coating or sheet-shaped print.
In the above-described methods, it is preferable to achieve crosslinking with a crosslinking agent, which reacts with hydroxyl groups in the resin of the present invention, upon formation of a printed coating (in the case of a printing ink), an adhesive coating (in the case of an adhesive) or a prime coat (in the case of a primer). By such crosslinking, the coating or coat can be converted into a coating or coat having excellent durability against wetness or dew under ordinary conditions. Further, upon removal of the coating, the crosslinking promotes penetration of alkaline water into the coating and moreover, keeps the integrity of the coating after its removal, so that separation of the coating from the separating solution can be facilitated. Examples of such a crosslinking agent can include polyisocyanate compounds, polyepoxy compounds, amino resin, and glutaraldehyde.
Specific examples of the polyisocyanate compounds can include biurets of diisocyanates such as tolylene-diisocyanate (TDI), 4,4xe2x80x2-diphenylmethanediisocyanate (MDI), xylene-diisocyanate (XDI), hexamethylenediisocynate (HMDI) and isophoronediisocyanate (IPDI); adducts of the above-described diisocyanates with trimethylolpropane and the like; polyisocyanates such as trimers of diisocyanates; blocked polyisocyanates obtained by blocking polyisocyanates with alcohols (ethanol, isopropanol, n-butanol), phenol derivatives (phenol, nitrophenol), lactams (caprolactam) or active methylene compounds (diethyl malonate, methyl acetoacetate, ethyl acetoacetate, acetylacetone). In addition, carbodiimide crosslinking agents, which are available from the above-described isocyanates, and their derivatives can also be mentioned.
Illustrative of the polyepoxy compounds are triglycidyl isocyanurate, 3,4-epoxycyclohexylmethyl, and 3,4-epoxy-cyclohexanecarboxylate. Illustrative of the amino resins are dimethylol urea, methylol melamine, butylated methylol melamine. Besides, glutaraldehyde, silane coupling agents, tetraisopropyl titanate, acetyl acetotitanate, aluminum alcoholates, and aluminum chelates can also be mentioned.
Examples of the article to which the resin composition of the present invention is applied can include various plastic products such as containers and wrapping films, glass products, wooden products, and iron products. No particular limitation is imposed on the shapes of these articles. The print or coat amount may preferably range from 0.01 to 20 xcexcm in terms of dry coat thickness. A print or coat amount smaller than 0.01 xcexcm is difficult to achieve uniform printing or coating, while a print or coat amount greater than 20 xcexcm is uneconomical and impractical.
The printing or coating of the resin composition of the present invention onto the surface of the article can be conducted by a conventional method, and no particular limitation is imposed on the printing or coating method. For example, gravure printing, letterpress printing (flexography), roll coating, reverse roll coating (transfer coating), spray coating or the like can be used. Formation of a resin coating and printing or the like may be conducted either as a continuous (in-line) system or as separate steps. No particular limitation is imposed on the printing or coating of an article in the present invention. Printing or coating can be effected directly onto a surface of an article, or can be effected onto a desired coating adhered on the surface of the article.
In the method of the present invention, removal of a coating of the resin composition of the present invention can be effected, for example, forming a plastic film, plastic container or the like, on which characters, a pattern and/or the like have been printed or adhered in accordance with the present invention, into small fragments by a shredder or the like as needed, dipping the plastic film, plastic container or the like or the small fragments thereof in alkaline water of low concentration to subject methyl acrylate units in the coating to saponification treatment, and allowing the coating to swell with the alkaline water. For example, a coating printed on an article or a sheet-shaped print adhered on an article can be removed preferably by placing the article in alkaline water and heating it to 70 to 80xc2x0 C. under gentle stirring. It is preferred to remove a resin coating, a coating printed on the resin coating or a sheet-shaped print adhered on the resin coating by using alkaline water of caustic soda, sodium carbonate or the like as a saponifying agent and causing the resin coating to swell. Upon saponification, an organic solvent such as methanol or ethanol may be added to promote a saponification reaction.